1. Field of the Invention
The present invention relates to a projection exposure apparatus for transferring a pattern of a photo mask or a reticle onto a photosensitive substrate, which is employed in a photolithography process of manufacturing, e.g., a semiconductor device, a liquid crystal display device or a thin-film magnetic head, etc.
2. Related Background Art
Recently, an integration degree of the semiconductor device has increased more and more, and making a circuit pattern hyperfine has been demanded. For this purpose, however, it is required that a resolution of the projection exposure apparatus be enhanced. One of methods of enhancing the resolution is to increase a numerical aperture of a projection optical system. If the numerical aperture is increased, however, a focal depth of a projection image decreases. Further, a process of manufacturing the semiconductor device becomes complicated, and there is a tendency of enlarging a level difference within the circuit patterns formed on the wafer. For this reason, an average surface is obtained corresponding to a level-difference structure within the on-the-wafer shot areas onto which the reticle patterns are transferred. The wafer is moved relatively to an imaging surface of the projection optical system corresponding thereto, whereby the entire average surface of the shot areas substantially aligns with the imaging surface of the projection optical system. That is, the entire average surface is set within the focal depth. A conventional projection exposure apparatus is equipped with a focus mechanism for adjusting a position (focus position) of an exposure surface of the wafer in the optical-axis direction of the projection optical system and a tilt mechanism for adjusting a tilt angle of the exposure surface thereof. Hereinafter, an operation of adjusting the tilt angle of the above exposure is termed [leveling].
FIG. 3A illustrates one example of the conventional projection exposure apparatus. Referring to FIG. 3A, a reticle 2 is illuminated with beams of illumination light from an illumination optical system 1. An image of a pattern on the reticle 2 is transferred onto the shot area on a wafer 5 held on a wafer table 4 via a projection optical system 3. A movable mirror 6 is attached to an edge portion of on the wafer table 4. The movable mirror 6 and an XY interferometer 8 cooperate to measure a position of the wafer table 4 within a plane (hereinafter called an XY plane) perpendicular to the optical axis of the projection optical system 3.
The wafer table 4 is placed on an X-stage 10 through three stretchable/contractible fulcrums 9A-9C composed of piezo-elements or the like. The X-stage 10 is placed on a Y-stage 11. The Y-stage 11 is placed on a base 12. The three fulcrums 9A-9C and a driving unit within the X-stage 10 are combined to constitute a tilt mechanism. The wafer table 4 is tilted by this tilt mechanism, thereby effecting the leveling of the wafer 5. Further, the three fulcrums 9A-9C are stretched and contracted by the same quantity, and a focus position of the wafer 5 is thereby minutely adjusted (focusing). Further, the X-stage 10 is moved through the driving unit 13. The Y-stage 11 is moved through a driving unit 14. Thus, the pattern image of the reticle 2 is exposed on each shot area on the wafer 5.
Further, a gate-shaped column 15 is embedded into the base 12. The illumination optical system 1 is fixed to a first beam 15a of the column 15. A table unit 16 for supporting the reticle 2 is formed on a second beam 15b of the column 15. The projection optical system 3 is fixed to a third beam 15c of the column 15. The XY interferometer 8 is secured to a strut portion of the column 15. The movable mirror 6 is irradiated with a laser beam from the XY interferometer 8, and, at the same time, a reference mirror 7 fixed to a side surface of the projection optical system 3 is also irradiated with the laser beam. Coordinates of the wafer table 4 are thus measured.
Attached further to a side surface of a lower portion of a lens barrel of the projection optical system 3 are an alignment system 17 based on an off-axis method and a surface detection sensor 18 for detecting a position and a tilt angle of the exposure surface of the wafer 5. The alignment system 17 is defined as an optical system for detecting positions of alignment marks on the wafer 5 by, e.g., an image processing method or a method of performing the irradiation of the laser beam for detecting the position. The surface detection sensor 18 is constructed of, as disclosed in, e.g., U.S. Pat. No. 4,558,949, a focus sensor for detecting a deviation quantity from the imaging surface of the projection optical system 3 at a predetermined point on the exposure surface of the wafer 5 and a leveling sensor for detecting the tilt angle of the exposure surface of the wafer 5 with respect to the imaging surface.
Then, hitherto, the exposure surface of the wafer 5 substantially aligns with the imaging surface of the projection optical system 3 by adjusting the stretching/contracting quantities of the fulcrums 9A-9C on the basis of output signals of the surface detection sensor 18. Note that U.S. Pat. Nos. 4,504,144 and 4,676,649 disclose a mechanism for driving the wafer table 4 and one example of a control method thereof.
FIG. 3B illustrates another example of the conventional projection exposure apparatus. A different point of this conventional example from the former conventional example of FIG. 3A is that the fulcrums 9A-9C constituting the tilt mechanism are provided on a wafer table 4B. More specifically, referring to FIG. 3B, a wafer table 4B is mounted on the X-stage 10. A wafer holder 4A is placed on the wafer table 4B through the three fulcrums 9A-9C. The wafer 5 is held on this wafer holder 4A. Further, the movable mirror 6 is fixed to one edge of the wafer table 4B. The XY interferometer 8 always measures a position of the wafer table 4B. Other configurations are the same as those in the example of FIG. 3A.
Accordingly, in the conventional example of FIG. 3B, the stretching/contracting quantities of the fulcrums 9A-9C defined as the tilt mechanism are adjusted based on the detected result of the surface detection sensor 18, thereby adjusting a focus position and a tilt angle in such a way that the wafer 5 is integral with the wafer holder 4A. In the conventional example of FIG. 3A, when operating the tilt mechanism, the tilt angle of the movable mirror 6 is also varied. In contrast with this, according to the conventional example of FIG. 3B, even when operating the tilt mechanism, the tilt angle of the movable mirror 6 does not change. This point is a big difference therebetween.
Now, in the case of performing the above-stated leveling, the wafer 5 makes almost no displacement in X- and Y-directions even when the fulcrums 9A-9C are stretched and contracted within a range where a trace quantity of variation in the tilt angle of the wafer 5 is seen. As a matter of fact, however, it may happen that the wafer 5 is shifted by a trace quantity in the X- and Y-directions with the stretch/contraction of the fulcrums 9A-9C due to a backlash and a deformation of the tilt mechanism. If an exposure takes place in a state where the wafer 5 is thus shifted in the X- and Y-directions, an overlay accuracy of the projected image of the pattern of the reticle 2 with respect to the shot area on the wafer 5 drops down, resulting in such a disadvantage that a yield is deteriorated.
Especially in the conventional example of FIG. 3B, the movable mirror 6 and the fulcrums 9A-9C are fixed onto the wafer table 4B, and, hence, the XY interferometer 8 is incapable of measuring the X- and Y-directional deviation quantities of the wafer 5 that are caused due to the stretch and contraction of the fulcrums 9A-9C. This is a factor for deteriorating the locating accuracy.
On the other hand, in the conventional example of FIG. 3A, there is no movable portion between the movable mirror 6 and the wafer 5. The XY interferometer 8 is capable of detecting the X- and Y-directional deviation quantities of the wafer 5 that are produced due to the backlash and deformation of the tilt mechanism. In the configuration of FIG. 3A, however, the X- and Y-directional deviation quantities are inevitably produced in terms of the system configuration in addition to the backlash and deformation of the tilt mechanism. This deviation quantity is classified into the following four error components.
(1) Interferometer-cosine Error
The movable mirror 6 is tilted, and the optical path of the laser beam of the XY interferometer 8 is consequently tilted. This error is caused due to a variation in terms of a relationship between a read value of the XY interferometer 8 and an intra XY-plane position of the wafer 5.
(2) Abbe Error
This error is caused by effecting the leveling in a state where a Z-directional position of the laser beam of from the XY interferometer 8 is different from a Z-directional position of the exposure surface of the wafer 5, i.e., in a state where the laser beam from the XY interferometer 8 does not satisfy the Abbe condition in the Z-direction.
(3) Wafer-cosine Error
This error is produced due to a variation in length of a line segment obtained by projecting a line segment from the movable mirror 6 to an exposure position on the wafer 5 onto the XY plane, which variation occurs with the leveling operation.
(4) Movable Mirror Tilt Error
If a reflecting surface of the movable mirror 6 is set with a tilt to the wafer table 4 (or 4B) from the perpendicular plane, this error is caused due to a variation in distance from a reflecting point of the laser beam from the movable mirror 6 to the exposure position on the wafer with a Z-directional displacement of the movable mirror 6 when effecting the leveling.
Magnitudes of the above error factors are minute enough not to present a problem hitherto. However, with an advancement of a hyperfine structure in terms of design rules of the semiconductor device, etc., these magnitudes thereof are not ignorable any more. The magnitudes of these error factors change depending on a variation quantity of the tilt angle due to the leveling, the tilt angle of the movable mirror 6 with respect to the wafer table 4 (or 4B), a distance from the movable mirror 6 to the exposure shot, a length of the optical path of the XY interferometer 8 and an Abbe offset quantity (deviation quantity which is a factor of the Abbe error) of the exposure surface of the wafer with respect to the XY interferometer 8. Error quantities based on those error factors are obtainable by calculation, and, therefore, after completing the leveling operation and the locating within the XY plane of the wafer 5, those error quantities can be corrected by minutely adjusting the X- and Y-directional positions of the wafer table 4 (or 4B). This, however, implies nothing but to locate the wafer table 4 (or 4B) at two stages, which is undesirable because of decreasing a throughput of the exposure process.
Further, when the travel guide surface of XY-stage has undulations, and the XY-stage is driven, a movable mirror on a focus leveling stage may be tilted, which is so-called rolling and pitching. Also in this case, a measured value by a laser interferometer undesirably includes a so-called Abbe error.
If the exposure of circuit patterns is repeated with the Abbe error, a proper locating accuracy is not ensured. A semiconductor integrated circuit with a higher integration degree cannot be manufactured.